Vacuum tube circuit



NOV. 16,1937. 5, BLACK 2,098,950

VACUUM TUBE CIRCUIT Filed Sept. 25, 1934 PLATE OF LAST TUBE- GRID or FIRST TUBE (TR/ODE, pnvrooz, OR OTHER TUBE) AAA M vii MMWMWV M.

/N 1 5 N TOR H. 5 BLACK A TTORNEY' Patented Nov. 16, 1937 r UNITED STATES PATENT OFFICE VACUUM TUBE CIRCUIT Harold S. Black, Elmhnrst, N. Y., assignor to BellTelephone Laboratories, Incorporated, New Yer-k, N. Y., a corporation of New York Application September 25, 1934, Serial No. 745,420

12 Claims. (01. 179-171) This application is a continuation in part of mately the loop transfer constant, as pointed my copending applications, Serial No. 606,871 out in the above mentioned publications and cofiled April 22, 1932, for Wave translation system pending applications. The unidirectional plate and Serial No. 663,317 filed March 29, 1933, for potential for the last stage may be supplied by Wave translation systems. a source of voltage included in one diagonal of 5 This'invention relates to wave translating sysa bridge network which has the feed-back path tems, particularly systems involving electric space in its other diagonal and the bridge maybe discharge devices as, for example, wave amplifybalanced for direct current sothat unidirectional ing vacuum tube devices. potential from the plate potential supply source 0 An object of the invention is to control transis suppressed from the feed-back path and conmission properties of such systems as, for insequently from the grid to which the alternating stance, to control impedance relations, modulapotential feed-back is applied. The plate-cathtions, Wave reflection, cross-talk, transmission ode path in the last tube of the amplifier may efficiency, space currents,- gain-frequency relabe in one ratio or balancing arm of the bridge,

' tions or singing tendencies involved in such systhe outgoing load circuit may be in the diagonal 5 tems. V with the plate current supply source, and the Afeatureof the invention relates to effecting load circuit may include an amplifier output such control by feed-back in the system. If detransformer matching the output impedance of sired, thefeed-back-may benegative feed-back or the amplifier and the load impedance.

feed-back that reduces gain, such feed-back in- The direct current plate-cathode impedance 2 creasing stability of operation and reducing disin the tube may diiier Widely from the altertortion as pointed out in the above mentioned nating current value for example, beinga'pproxicopending applications and in my article on mately twice the alternating current value in Stabilized Feed-back Amplifiers published in usual triodes and of the order of one-tenth of Electrical Engineering, January 1934, pages 114- the alternating current value in the case of usual 25 120 and in the January 1934 issue of The Bell power'pentodes. In such cases, the bridge will System Technical Journal, pages 1-18. be unbalanced for alternating current and feed- It is also an object of the invention'to faciliback will change the amplifier output impedance. tate application of feed-back in systems such as As disclosed in the above mentioned copendthose referred to above. ing application, Serial No. 663,317, the negative 30 A further objectof the invention is to stabilize feed-back tends to make the amplifier output space current of electric space discharge devices impedance independe t o the impedance Va e as, for example, wave amplifying Vacuum tube of the bridge arm containing the space dischar e 1 devices. a path, i. e., the R0 arm, and changes the amplifier In one specific aspect the invention is a feedoutput impedance to render it less different from 35 back amplifier with conjugaterelation of plate the value it would have if the bridge Were recurrent supply source and feed-back path, so balanced by adjustment of the impedance of the that the grid to which alternating potential is R0 a m. S disclosed in that pp eh,' e fed back will not receive undesired unidirectional ega e ee k is made large, s p a ce 40 potential, notwiths'tandingomission of the stopstabilizing effect is powerful, in Other Wo ds, 40 ping condenser usually employed in the feede un y s d negligibly Small in p en back path to prevent the undesired application to the loop transfer constant, the amplifier outof unidirectional potentialtdthe grid. It is often Put impedance can be made to approach of apadvantageous to avoid use of-the stopping conproximately equal the Value it o d ave if the denser since its use may give riseto troublesome bridge were balanced by adjustment of the R0 45 phase shift problems-at low frequencies because of arm, and to become substantially independent its impedance and at high frequencies because of O t e impeda ce of that arm. Thus, as disits capacity to ground. closed in that application, the output trans- The amplifier may be, for example, a negative former ratio can be given the value proper for feed-back amplifier. Theloop transfer constant, a c g t e a p fi u p peda e and 50 (i. e; the transfer constant for propagation once th a impedancearoun'dthe feed-back loop) may best) large that In the case of the triodes, the tube then ordiunity is negligible in comparison, in which case narily will work not into a rnatching impedance the feed-back divides the amplification and but into an impedance approximately twice as modulationproduced bytlie amplifierbyapproxlgreat (or equal to its direct current impedance) 55 which has been found a favorable condition of operation, for example, for reducing modulation.

In the case of the power pentodes, it is ordinarily desirable that the tube work into an impedance only a fraction, for instance a fifth or a sixth, of its matching impedance, and moreover, when the bridge is adjusted for the direct current balance, the feed-back ordinarily lowers the amplifier output impedance by approximately the ratio of the alternating current and direct current impedances of the tube or of the bridge arm containing the space discharge path. Consequently, in the case, for instance, of cable carrier or open wire carrier systems with the amplifier output transformer required to match the'amplifier output impedance to a 140 ohm line or a 600 ohm line, respectively, the use of the direct'current bridge balance with the power pentode does not necessitate an impracticable or inconvenient transformer ratio, even though the impedance match is required over a wide frequency band.

With the space current supply source in the load diagonal of the bridge, ordinarily the source is very efliciently used, nearly all of :its voltage being applied across the plate and cathode and but little of its energy being dissipated in the bridge impedances other than the plate-cathode impedance in the tube.

By unbalancing the bridge for direct current and controlling the degree of unbalance, the transmission of unidirectional voltage from plate current supply source to grid through the feedback path of the amplifier can be controlled as may be desired. For example, the direct current impedance of a triode in general increases with hours of use of the cathode, and, if desired, the space current decrease that would'result can be avoided by having negative bias of the grid decrease in response to unbalancing of the bridge for direct current by the increase of the direct current impedance of the tube as referred to more in detail hereinafter.

If desired, the space current compensation thus effected by the unbalance can be measured, for example as indicated hereinafter, for determining when the tube is in such condition that it should be retired from service.

Other objects and aspects of the invention will be apparent from the following description and claims.

Fig. 1 of the accompanying drawing shows schematically a vacuum tube amplifier circuit embodying a form of the invention;

Figs. 2 and 3 show examples of grid biasing means that may be used in connection with the circuit of Fig. 1;

Fig. 4 shows means suitable for connection in the feed-back path of the amplifier of Fig. l for preventing application of undesired grid bias by suppressing transmission through that path of unidirectional voltage resulting from variation of space discharge path impedance for direct current; and V I Fig. 5 shows a modification of the circuit of Fig. 1 I

For any given frequency, the impedance of a network between any two points is considered the ratio thata voltage applied across the points from an external network would bear to the internetwork current.

The amplifier of Fig. 1 comprises one or more stages of vacuum tubes,'as for example, three stages. bridge 2 connects the final stage to the feedback path and to amplifier output transformer T.

It has a feed-back path f. An output An input bridge 3 connects the first stage to amplifier input transformer T, and to the feed-back path. The feed-back path is in one diagonal of the input bridge and in one diagonal of the output bridge. In the input bridge the other diagonal includes the input transformer, and in the output bridge the other diagonal includes the output transformer or load and the plate current supply source B, which may be any suitable source of unidirectional voltage.

The input and output transformers may connect the amplifier in a cable carrier telephone or other circuit, the amplifier then being used for example for simultaneously amplifying a plurality of telephone messages, all transmitted together over the cable carrier circuit from multiplex carrier transmitting apparatus (not shown). Transformer T may be considered part of the receiving circuit or load, the amplifier output impedance being considered the impedance facing the primary or high impedance winding. of transformer T.

The amplifier may be of the general type of those shown for example in Figs. 5, 56 and 5'? of the above mentioned copending application, Serial No. 606,871 and Fig. 2 of the above mentioned publications, each of those amplifiers having a feed-back path connected between output and input bridges. The feed-back may be negative feed-back, and the amount of feed-back may be large. the absolute or scalar value of the loop transfer constant being, for example, of the order of 50 or in the utilized frequency range, as in the case of the negative feed-back amplifier of Fig. 57 or Fig. 2 just mentioned.

The arms of the output bridge and their direct current impedance values for a balanced condition of the bridge for direct curent are represented by Rdc, KRdc, R and KR, Rdc being the arm including the space discharge path of the tube (or tubes) of the final stage of the amplifier. The alternating current impedance of this arm is designated R0. The arms of the input bridge, and alternating current impedance values for alternating current balance of the bridge, are represented by R'o, KRo, R and K'R'. The bridge arms may be resistances or any suitable imped ances. Ordinarily it is desirable, for avoiding undue loss in transmission to the load, that, as regards alternating current impedances, KRdc be small and R large compared to the load impedance; and consequently the impedance into which the tube works is not materially different. from the load impedance or high side impedance of the output transformer.

It is usual to employ a stopping condenser in the feed-back path. The use of the stopping condenser may give rise to troublesome phase-shift problems, and can be avoided, in accordance with the invention, by balancing the output bridge for direct current, so source B can not apply unidirectional potential to the grid through ,1, notwithstanding omission of the stopping condenser from path 1 so that there is a conductive connection from source B to the grid through While undesired application of unidirectional voltage from the space current supply source to the grid through the feed-back path might, in some cases, be avoided by employing a transformer in the feed-back path, connection of a transformer in the feed-back path is often objectionable, for example as producing troublesome singing tendency in the amplifier, either within the utilized frequency range or outside of that range. The suppressionof transmission of unidirectional voltage from-thespacecurrent suppl-ysource to the grid through the feed-back path by balancing the output bridge for direct current in accordance with the invention, is useful also in cases in which a phase reversal is introduced in the feed-back, without a transformer, for example by reversing the feed-back leads as in E. Peterson Patent No. 15955327, issued April 24, 1934. The reversal of the feed-back leads maybe desirable, for example, due to the number of stages in the amplifier being even, or because of the phase-frequency characteristic of the feed-back loop.

The tube (or'tubes) of the final stage of the amplifier may be of any suitable type, as for-example a heater type triode or power pentode. If the tube be a triode, Rdc ordinarily is approximately 2 R0. If the tube be a power pentode, its D. C. impendance Rdc may be, for example, a tenth of its alternating current impedance Re. In either case,

if each of the arms R, KR and KRdc of the output bridge has its alternating current impedance equal to its direct current impedance, the bridge, when balanced for direct current, will be unbalanced for alternating current.

Nevertheless, the amplifier output impedance then can be matched to the load impedance by making the primary-to-secondary impedance ratio of the transformer where c is the impedance of the cable or circuit facing the secondary winding. As indicated above, though K is small, doubling the alternating current impedance of the KRdc arm will approximately double the amplifier output impedance, and reducing the alternating current impedance of the KRdc arm 90 per cent will reduce the amplifier output impedance approximately 90 per cent. V

If desired negative grid bias for any of the tubes of the amplifier of Fig. 1 may be supplied by the usual bias resistor and by-pass condenser in the cathode lead common to the external cathodegrid'circuit of the tube (or tubes) whose grid structureis to be biased and an appropriate one (or appropriate ones) of the external cathodeanode circuits offthe tubes. For example, asindicated'by Fig. '2',the'bias may be supplied by resistor 4 and by-pass condenser 5 connected in cathode leadL. By making condenser 5 of large capacity, alternating current feed-back from resistor 4 is reduced. The impedance of the resistor and condenser will be included in R0 and Rdc, since they are in the bridge arm with the space discharge path.

If desired negative grid bias for any of the tubes of the amplifier of Fig. 1 may be supplied by the usual grid-bias battery connected at a suitable place in the external circuit between the cathode and grid of the tube, as for example,battery'C in the cathode lead L as indicated by Fig. 3.

If due to variation in Rec, a unidirectional voltage is applied to the feed-back path in the circuit of Fig. 1, and for any reason it is desired not to race back direct current, then a direct current filter, (i. e. a filter offering high attenuation to all very low, frequencies) may be used in the feedback path f'to suppressthis voltage, asindicated forexample by Fig. 4,.showing a filter comprising high resistance 1 and'by-pass condenser 8 connected in parallel to each other in series in the feed-back path '1.

In the circuit of Fig. 1, instead of balancing the output bridge for direct current,.itcan be so unbalanced as to supply unidirectional grid biasing voltage through I. For example, increasing the impedance of the KRdc arm above the value required for direct current balance of the bridge, or reducing the impedance of the KR arm below the value-required for direct current balance, produces negative biasing potential on the grid. This bias method-can automatically compensate for changes in Erie. For example, if Rdc increases due tothe usual increase of tube resistance with the number of hours the tube has been in use, the negative'bias'is automatically reduced. This assumes that if the grid to which the direct current from the output bridge is applied is a grid of a stage other than the last stage, the interstage couplings will transmit the direct current so that it will be propagated to the grid structure of the last stage.

Fig. 5 shows a modification of the circuit of Fig. 1 in which a tube 10 has input and output bridges connecting it between input and output transformers, as in Fig. 1, but a choke coil l I and switch l2, adapted for testing tube Iii as described below, are included in series in path l3 which is in the bridge diagonals opposite those that include the transformers. The choke coil prevents feed-back of alternating current through [3. If desired, an alternating current feed-backpathcan be provided. For example, a feed-back path I can be connected to produce negative feed-back from a bridge network 2' on the secondary or low impedance side of the output transformer to a bridge network 3 on the primary or low impedance side of the input transformer. feed-back throughbridges outside of the amplifier input and output transformers is disclosed in Figs. 38 and'73, for example, of the above mentioned copending application Serial No. 606,871. If desired, either or both of the bridges 2 and 3' can be omitted, the feed-back voltage for transmission through I' being obtained, for example, from across the low impedance winding of the output transformer and applied, for example, across the low impedance winding of the input transformer, as disclosed in Fig. 69, for example, of the copending application just mentioned.

In Fig. 5 the battery B is shown in shunt to the primary winding of transformer T, instead of in series with it as in Fig. 1. The usual alternating current choke l5 and direct current stopping condenser l6 prevent the battery from shunting alternating current around the winding or transmitting direct current through the winding. A milliammeter 20 is provided for reading the direct current space current. It is shown in the bridge diagonal that includes the B battery, since the current flowing from the battery through R and KR is negligibly small. However, if desired the milliammeter may be connected, for example, in the KRdc arm.

In normal operation, battery 0 may provide the normal negative grid bias required, switch I2 may be closed, and the bridge 2 may be balanced for direct current, substantially no grid bias voltage being supplied from battery B through path I3. Then as Rdc tends to increase with long use of the tube, the resulting tendency to unbalance of the bridge tends to apply positive potential to the grid from battery B through path i3, thus tending to lower the negative potential of-the grid so as to maintain the tube impedance and space current constant. To tell whether the tube has deteriorated to such extent that it should be retired from service, switch l2 This can be opened and the resulting change in the milliammeter reading observed.

What is claimed is:

1. A space discharge device having a space current supply source and feed-back path, and circuits connecting the source and the path in conjugate relation to each other with respect only to steady unidirectional voltage and supplying a current from the source to the deviceand a current from the device to the path.

2. A wave translating device having a space discharge path and a space current supply source and having a circuit connected from output to input, and means connecting said source and circuit in a bridge network whose degree of balance for direct current differs from that for alternating current and controls transmission of steady unidirectional voltage from the space current supply source through said circuit to the input.

3. A space discharge device having a space discharge path, a space current supply source connected to said discharge path and a feed-back path connected to said discharge path, and a bridge circuit whose degree of balance for direct current controls transmission of steady unidirectional voltage from the space current supply source to the feed-back path, said bridge circuit having said space discharge path as a balancing arm of the bridge circuit and having said space current supply source and said feed-back path in diagonals or" the bridge circuit that are conjugate at balance of the bridge circuit.

4. A space discharge device having a space discharge path, a space current supply source'conn'ected to said discharge path and a feed-back path connected to said discharge path, and a.

bridge circuit whose degree of balance for direct current controls transmission of steady unidirectional voltage from the space current supply source to the feed-back path, said bridge circuit having said space discharge .path in a balancing arm of the bridge circuit and having said space current supply source and said feed-back path in diagonals of the bridge circuit that are conjugate at balance of the bridge circuit, and having a load circuit for said device in the branch that includes said space current supply source.

5. A space discharge device having a space discharge path, a space current supply source and a grid bias circuit all in a bridge circuit whose degree of balance for direct current controls transmission of steady unidirectional voltage from t e space current supply source to the grid bias circuit and one of whose balancing arms includes said space discharge path and has substantially different impedance for alternating, current than for direct current. ,7

6. A space discharge device having an anode, a cathode and space discharge control means, a source of space current therefor, a circuit for supplying biasing potential to said control means, and means connecting said biasing circuit, said space current source and the anode-cathode space discharge path of said device in abridge circuit whose degree of balance for direct current controls transmission of steady unidirectional voltage from said space current source to said biasing circuit, with the spacedischarge path a balancing arm of the bridge circuit and the space current source and the biasing circuit in diagonals of the bridge circuit that are conjugate at balance of the bridge circuit.

'7. A space discharge device having a space discharge path, a space current supply source and a grid bias circuit all in a bridge circuit Whose degree of balance for direct current controls transmission of steady unidirectional voltage from the space current supply source to the-grid bias circuit and one of whose balancing arms includes said space discharge path, said grid bias circuit comprising a choke coil and a switch in series therein.

8. A wave translating device having a space discharge path and a space current supply source and having a circuit connected from output to input, and means connecting said source and circuit in a bridge network whose degree of balance for direct current controls transmission ofsteady unidirectional voltage from the space current supply source through said circuit to the input, the impedance of a balancing arm of the bridge network being determined principally by said space discharge path.

9. The method of operating a retroactive space discharge device having space current supply source and a feed-back path in abridge circuit, which comprises determining the impedance of one of the bridge balancing arms principally by the impedance of the space path of the device, and controlling the transmission of steady unidirectional voltage from the space current supply source to the feed-back path by control of degree of balance of the bridge circuit for direct current.

ii). The method of operating a space discharge device having a space discharge path, a space current Supply source'and a grid biasing circuit all in abridge circuit, which comprises controlling the transmission of steady unidirectional voltage from the space current supply source to the grid biasing circuit by control of degree of balance of the bridge circuit for direct current and maintaining the impedancev of one of the bridge balancing means different for alternating current than for direct current.

11. A wave translating device having a space discharge path and a space current supply source and having a iced-back path providing negative feed-back of alternating voltage with the transfer constant for propagation once around the feedback loop large compared to unity, and a bridge circuit whose degree of balance for direct current controls transmission of steady unidirectional voltage from the space current supply source to the feed-back path, said bridge circuit having said space discharge path in a balancing arm of the bridge circuit and having said space current supply source and said feed-backv path in diagonals of the bridge circuit that are conjugate at balance of the bridge circuit.

12. A circuit comprising a wave translating device having a space discharge path and a space discharge control grid, a space current source for supplying space current to said path, means for supplying to said grid steady unidirectional biasing potential variable in response to changes in impedance of said space discharge path and substantially zero when said impedance has its normal value, means operable for rendering said first mentioned means ineffective without interrupting said space current, and means for meas- 

